1. Field of the Invention
The present invention relates to a probe support for test fixture of printed circuit artworks (PWA).
2. Description of Prior Art
As known, the PWA test fixtures enable the electrical connection of a plurality of nodes of a circuit implemented on a circuit board to an automatic test system. Such fixtures generally comprise:
a holding station for the PWA to be tested; PA1 an element provided with a suitably arranged plurality of probes which are forced to contact prearranged soldering spots of the printed circuit board; PA1 a connector whose pins are electrically connected to the probes with the well known "wire wrapping" technique; PA1 mechanical or pneumatic actuation means forcing the electrical contact between probes and soldering spots of the printed circuit board.
It is clear that specific probe support elements are to be provided and suitable electrical connections between probes and connector pins are to be arranged for each different PWA to be tested. In the prior art, to avoid the need for several probe supports, universal probe supports, that are provided with a probe matrix, have been suggested; such probes being selectively actuated according to the configuration of the board to be tested. This type of solution is, for instance, disclosed in U.S. Pat. No. 4,352,061. However the manufacturers of PWA's still prefer, for mass production suitably arranged probe supports, in view of the high cost of the universal tips and the maintenance costs necessitated by the unavoidable probe deterioration which reduces the reliability of the electrical contact between the probes and the soldering spots, after a certain number of test operations. The embodiment of specific probe supports is implemented by using insulating material boards supporting the probes and having a distribution of the holes identical to the ones corresponding to the soldering spots of the PWA to be tested. Such probe-supporting insulated boards are obtained by means of working bases identical to the ones used for manufacturing the printed circuit boards, and utilized in the assembly of the PWA's to be tested as disclosed in U.S. Pat. No. 4,132,948. In setting up a fixture for testing PWA's a great amount of work and time is spent for the wiring harness between the probe pins and the corresponding pins of the connector. In order to facilitate and speed up the wiring harness operations, U.S. Pat. No. 4,099,120 suggests arranging the connectors on the probe support so that the pins of the connector and the ones of the probes used for the wiring harness are on the same plane. According to this patent, the probe support comprises a unitary element independent of the remaining parts of the test fixture, and accordingly wiring harness operations can be easily automated. However such a probe support results in handling difficulties and requires careful handling during the wiring harness operation. In fact, the probe tips are not suitably protected and kept in a fixed rest position when the probe support is not mounted on the test fixture. A further disadvantage, occurring in the probe support disclosed in U.S. Pat. No. 4,132,948 and in test fixtures known in the art, is that there may not be the desired electrical contact between some of the contact tips of the probes and the corresponding soldering spots of the printed circuit board, during the testing phase. In fact, when the probes are forced against the board to be tested, the contact tips of some of such probes deviate from their axis owing to bulging of the soldering spots, so the electrical contact with the pre-established node of the circuit to be implemented on the printed circuit board is not assured. In the worst case condition the deviated contact tip of the probe may cause a short circuit between the point to be tested and the contiguous one.
Such disadvantages are overcome by the prsent invention.